3    "Good for you, you've decided to clean the elevator!"
   4    - The Elevator, from Dark Star
   6Smack is the the Simplified Mandatory Access Control Kernel.
   7Smack is a kernel based implementation of mandatory access
   8control that includes simplicity in its primary design goals.
  10Smack is not the only Mandatory Access Control scheme
  11available for Linux. Those new to Mandatory Access Control
  12are encouraged to compare Smack with the other mechanisms
  13available to determine which is best suited to the problem
  14at hand.
  16Smack consists of three major components:
  17    - The kernel
  18    - Basic utilities, which are helpful but not required
  19    - Configuration data
  21The kernel component of Smack is implemented as a Linux
  22Security Modules (LSM) module. It requires netlabel and
  23works best with file systems that support extended attributes,
  24although xattr support is not strictly required.
  25It is safe to run a Smack kernel under a "vanilla" distribution.
  27Smack kernels use the CIPSO IP option. Some network
  28configurations are intolerant of IP options and can impede
  29access to systems that use them as Smack does.
  31The current git repository for Smack user space is:
  33        git://
  35This should make and install on most modern distributions.
  36There are three commands included in smackutil:
  38smackload  - properly formats data for writing to /smack/load
  39smackcipso - properly formats data for writing to /smack/cipso
  40chsmack    - display or set Smack extended attribute values
  42In keeping with the intent of Smack, configuration data is
  43minimal and not strictly required. The most important
  44configuration step is mounting the smackfs pseudo filesystem.
  45If smackutil is installed the startup script will take care
  46of this, but it can be manually as well.
  48Add this line to /etc/fstab:
  50    smackfs /smack smackfs smackfsdef=* 0 0
  52and create the /smack directory for mounting.
  54Smack uses extended attributes (xattrs) to store labels on filesystem
  55objects. The attributes are stored in the extended attribute security
  56name space. A process must have CAP_MAC_ADMIN to change any of these
  59The extended attributes that Smack uses are:
  62        Used to make access control decisions. In almost all cases
  63        the label given to a new filesystem object will be the label
  64        of the process that created it.
  66        The Smack label of a process that execs a program file with
  67        this attribute set will run with this attribute's value.
  69        Don't allow the file to be mmapped by a process whose Smack
  70        label does not allow all of the access permitted to a process
  71        with the label contained in this attribute. This is a very
  72        specific use case for shared libraries.
  74        Can only have the value "TRUE". If this attribute is present
  75        on a directory when an object is created in the directory and
  76        the Smack rule (more below) that permitted the write access
  77        to the directory includes the transmute ("t") mode the object
  78        gets the label of the directory instead of the label of the
  79        creating process. If the object being created is a directory
  80        the SMACK64TRANSMUTE attribute is set as well.
  82        This attribute is only available on file descriptors for sockets.
  83        Use the Smack label in this attribute for access control
  84        decisions on packets being delivered to this socket.
  86        This attribute is only available on file descriptors for sockets.
  87        Use the Smack label in this attribute for access control
  88        decisions on packets coming from this socket.
  90There are multiple ways to set a Smack label on a file:
  92    # attr -S -s SMACK64 -V "value" path
  93    # chsmack -a value path
  95A process can see the smack label it is running with by
  96reading /proc/self/attr/current. A process with CAP_MAC_ADMIN
  97can set the process smack by writing there.
  99Most Smack configuration is accomplished by writing to files
 100in the smackfs filesystem. This pseudo-filesystem is usually
 101mounted on /smack.
 104        This interface reports whether a subject with the specified
 105        Smack label has a particular access to an object with a
 106        specified Smack label. Write a fixed format access rule to
 107        this file. The next read will indicate whether the access
 108        would be permitted. The text will be either "1" indicating
 109        access, or "0" indicating denial.
 111        This interface reports whether a subject with the specified
 112        Smack label has a particular access to an object with a
 113        specified Smack label. Write a long format access rule to
 114        this file. The next read will indicate whether the access
 115        would be permitted. The text will be either "1" indicating
 116        access, or "0" indicating denial.
 118        This contains the Smack label applied to unlabeled network
 119        packets.
 121        This interface allows a specific CIPSO header to be assigned
 122        to a Smack label. The format accepted on write is:
 123                "%24s%4d%4d"["%4d"]...
 124        The first string is a fixed Smack label. The first number is
 125        the level to use. The second number is the number of categories.
 126        The following numbers are the categories.
 127        "level-3-cats-5-19          3   2   5  19"
 129        This interface allows a specific CIPSO header to be assigned
 130        to a Smack label. The format accepted on write is:
 131        "%s%4d%4d"["%4d"]...
 132        The first string is a long Smack label. The first number is
 133        the level to use. The second number is the number of categories.
 134        The following numbers are the categories.
 135        "level-3-cats-5-19   3   2   5  19"
 137        This contains the CIPSO level used for Smack direct label
 138        representation in network packets.
 140        This contains the CIPSO domain of interpretation used in
 141        network packets.
 143        This interface allows access control rules in addition to
 144        the system defined rules to be specified. The format accepted
 145        on write is:
 146                "%24s%24s%5s"
 147        where the first string is the subject label, the second the
 148        object label, and the third the requested access. The access
 149        string may contain only the characters "rwxat-", and specifies
 150        which sort of access is allowed. The "-" is a placeholder for
 151        permissions that are not allowed. The string "r-x--" would
 152        specify read and execute access. Labels are limited to 23
 153        characters in length.
 155        This interface allows access control rules in addition to
 156        the system defined rules to be specified. The format accepted
 157        on write is:
 158                "%s %s %s"
 159        where the first string is the subject label, the second the
 160        object label, and the third the requested access. The access
 161        string may contain only the characters "rwxat-", and specifies
 162        which sort of access is allowed. The "-" is a placeholder for
 163        permissions that are not allowed. The string "r-x--" would
 164        specify read and execute access.
 166        This interface allows process specific access rules to be
 167        defined. These rules are only consulted if access would
 168        otherwise be permitted, and are intended to provide additional
 169        restrictions on the process. The format is the same as for
 170        the load interface.
 172        This interface allows process specific access rules to be
 173        defined. These rules are only consulted if access would
 174        otherwise be permitted, and are intended to provide additional
 175        restrictions on the process. The format is the same as for
 176        the load2 interface.
 178        This contains the Smack logging state.
 180        This contains the CIPSO level used for Smack mapped label
 181        representation in network packets.
 183        This interface allows specific internet addresses to be
 184        treated as single label hosts. Packets are sent to single
 185        label hosts without CIPSO headers, but only from processes
 186        that have Smack write access to the host label. All packets
 187        received from single label hosts are given the specified
 188        label. The format accepted on write is:
 189                "%d.%d.%d.%d label" or "%d.%d.%d.%d/%d label".
 191        This contains the label processes must have for CAP_MAC_ADMIN
 192        and CAP_MAC_OVERRIDE to be effective. If this file is empty
 193        these capabilities are effective at for processes with any
 194        label. The value is set by writing the desired label to the
 195        file or cleared by writing "-" to the file.
 197        Writing a Smack label here sets the access to '-' for all access
 198        rules with that subject label.
 200You can add access rules in /etc/smack/accesses. They take the form:
 202    subjectlabel objectlabel access
 204access is a combination of the letters rwxa which specify the
 205kind of access permitted a subject with subjectlabel on an
 206object with objectlabel. If there is no rule no access is allowed.
 208Look for additional programs on
 210From the Smack Whitepaper:
 212The Simplified Mandatory Access Control Kernel
 214Casey Schaufler
 217Mandatory Access Control
 219Computer systems employ a variety of schemes to constrain how information is
 220shared among the people and services using the machine. Some of these schemes
 221allow the program or user to decide what other programs or users are allowed
 222access to pieces of data. These schemes are called discretionary access
 223control mechanisms because the access control is specified at the discretion
 224of the user. Other schemes do not leave the decision regarding what a user or
 225program can access up to users or programs. These schemes are called mandatory
 226access control mechanisms because you don't have a choice regarding the users
 227or programs that have access to pieces of data.
 229Bell & LaPadula
 231From the middle of the 1980's until the turn of the century Mandatory Access
 232Control (MAC) was very closely associated with the Bell & LaPadula security
 233model, a mathematical description of the United States Department of Defense
 234policy for marking paper documents. MAC in this form enjoyed a following
 235within the Capital Beltway and Scandinavian supercomputer centers but was
 236often sited as failing to address general needs.
 238Domain Type Enforcement
 240Around the turn of the century Domain Type Enforcement (DTE) became popular.
 241This scheme organizes users, programs, and data into domains that are
 242protected from each other. This scheme has been widely deployed as a component
 243of popular Linux distributions. The administrative overhead required to
 244maintain this scheme and the detailed understanding of the whole system
 245necessary to provide a secure domain mapping leads to the scheme being
 246disabled or used in limited ways in the majority of cases.
 250Smack is a Mandatory Access Control mechanism designed to provide useful MAC
 251while avoiding the pitfalls of its predecessors. The limitations of Bell &
 252LaPadula are addressed by providing a scheme whereby access can be controlled
 253according to the requirements of the system and its purpose rather than those
 254imposed by an arcane government policy. The complexity of Domain Type
 255Enforcement and avoided by defining access controls in terms of the access
 256modes already in use.
 258Smack Terminology
 260The jargon used to talk about Smack will be familiar to those who have dealt
 261with other MAC systems and shouldn't be too difficult for the uninitiated to
 262pick up. There are four terms that are used in a specific way and that are
 263especially important:
 265        Subject: A subject is an active entity on the computer system.
 266        On Smack a subject is a task, which is in turn the basic unit
 267        of execution.
 269        Object: An object is a passive entity on the computer system.
 270        On Smack files of all types, IPC, and tasks can be objects.
 272        Access: Any attempt by a subject to put information into or get
 273        information from an object is an access.
 275        Label: Data that identifies the Mandatory Access Control
 276        characteristics of a subject or an object.
 278These definitions are consistent with the traditional use in the security
 279community. There are also some terms from Linux that are likely to crop up:
 281        Capability: A task that possesses a capability has permission to
 282        violate an aspect of the system security policy, as identified by
 283        the specific capability. A task that possesses one or more
 284        capabilities is a privileged task, whereas a task with no
 285        capabilities is an unprivileged task.
 287        Privilege: A task that is allowed to violate the system security
 288        policy is said to have privilege. As of this writing a task can
 289        have privilege either by possessing capabilities or by having an
 290        effective user of root.
 292Smack Basics
 294Smack is an extension to a Linux system. It enforces additional restrictions
 295on what subjects can access which objects, based on the labels attached to
 296each of the subject and the object.
 300Smack labels are ASCII character strings, one to twenty-three characters in
 301length. Single character labels using special characters, that being anything
 302other than a letter or digit, are reserved for use by the Smack development
 303team. Smack labels are unstructured, case sensitive, and the only operation
 304ever performed on them is comparison for equality. Smack labels cannot
 305contain unprintable characters, the "/" (slash), the "\" (backslash), the "'"
 306(quote) and '"' (double-quote) characters.
 307Smack labels cannot begin with a '-'. This is reserved for special options.
 309There are some predefined labels:
 311        _       Pronounced "floor", a single underscore character.
 312        ^       Pronounced "hat", a single circumflex character.
 313        *       Pronounced "star", a single asterisk character.
 314        ?       Pronounced "huh", a single question mark character.
 315        @       Pronounced "web", a single at sign character.
 317Every task on a Smack system is assigned a label. System tasks, such as
 318init(8) and systems daemons, are run with the floor ("_") label. User tasks
 319are assigned labels according to the specification found in the
 320/etc/smack/user configuration file.
 322Access Rules
 324Smack uses the traditional access modes of Linux. These modes are read,
 325execute, write, and occasionally append. There are a few cases where the
 326access mode may not be obvious. These include:
 328        Signals: A signal is a write operation from the subject task to
 329        the object task.
 330        Internet Domain IPC: Transmission of a packet is considered a
 331        write operation from the source task to the destination task.
 333Smack restricts access based on the label attached to a subject and the label
 334attached to the object it is trying to access. The rules enforced are, in
 337        1. Any access requested by a task labeled "*" is denied.
 338        2. A read or execute access requested by a task labeled "^"
 339           is permitted.
 340        3. A read or execute access requested on an object labeled "_"
 341           is permitted.
 342        4. Any access requested on an object labeled "*" is permitted.
 343        5. Any access requested by a task on an object with the same
 344           label is permitted.
 345        6. Any access requested that is explicitly defined in the loaded
 346           rule set is permitted.
 347        7. Any other access is denied.
 349Smack Access Rules
 351With the isolation provided by Smack access separation is simple. There are
 352many interesting cases where limited access by subjects to objects with
 353different labels is desired. One example is the familiar spy model of
 354sensitivity, where a scientist working on a highly classified project would be
 355able to read documents of lower classifications and anything she writes will
 356be "born" highly classified. To accommodate such schemes Smack includes a
 357mechanism for specifying rules allowing access between labels.
 359Access Rule Format
 361The format of an access rule is:
 363        subject-label object-label access
 365Where subject-label is the Smack label of the task, object-label is the Smack
 366label of the thing being accessed, and access is a string specifying the sort
 367of access allowed. The access specification is searched for letters that
 368describe access modes:
 370        a: indicates that append access should be granted.
 371        r: indicates that read access should be granted.
 372        w: indicates that write access should be granted.
 373        x: indicates that execute access should be granted.
 374        t: indicates that the rule requests transmutation.
 376Uppercase values for the specification letters are allowed as well.
 377Access mode specifications can be in any order. Examples of acceptable rules
 380        TopSecret Secret  rx
 381        Secret    Unclass R
 382        Manager   Game    x
 383        User      HR      w
 384        New       Old     rRrRr
 385        Closed    Off     -
 387Examples of unacceptable rules are:
 389        Top Secret Secret     rx
 390        Ace        Ace        r
 391        Odd        spells     waxbeans
 393Spaces are not allowed in labels. Since a subject always has access to files
 394with the same label specifying a rule for that case is pointless. Only
 395valid letters (rwxatRWXAT) and the dash ('-') character are allowed in
 396access specifications. The dash is a placeholder, so "a-r" is the same
 397as "ar". A lone dash is used to specify that no access should be allowed.
 399Applying Access Rules
 401The developers of Linux rarely define new sorts of things, usually importing
 402schemes and concepts from other systems. Most often, the other systems are
 403variants of Unix. Unix has many endearing properties, but consistency of
 404access control models is not one of them. Smack strives to treat accesses as
 405uniformly as is sensible while keeping with the spirit of the underlying
 408File system objects including files, directories, named pipes, symbolic links,
 409and devices require access permissions that closely match those used by mode
 410bit access. To open a file for reading read access is required on the file. To
 411search a directory requires execute access. Creating a file with write access
 412requires both read and write access on the containing directory. Deleting a
 413file requires read and write access to the file and to the containing
 414directory. It is possible that a user may be able to see that a file exists
 415but not any of its attributes by the circumstance of having read access to the
 416containing directory but not to the differently labeled file. This is an
 417artifact of the file name being data in the directory, not a part of the file.
 419If a directory is marked as transmuting (SMACK64TRANSMUTE=TRUE) and the
 420access rule that allows a process to create an object in that directory
 421includes 't' access the label assigned to the new object will be that
 422of the directory, not the creating process. This makes it much easier
 423for two processes with different labels to share data without granting
 424access to all of their files.
 426IPC objects, message queues, semaphore sets, and memory segments exist in flat
 427namespaces and access requests are only required to match the object in
 430Process objects reflect tasks on the system and the Smack label used to access
 431them is the same Smack label that the task would use for its own access
 432attempts. Sending a signal via the kill() system call is a write operation
 433from the signaler to the recipient. Debugging a process requires both reading
 434and writing. Creating a new task is an internal operation that results in two
 435tasks with identical Smack labels and requires no access checks.
 437Sockets are data structures attached to processes and sending a packet from
 438one process to another requires that the sender have write access to the
 439receiver. The receiver is not required to have read access to the sender.
 441Setting Access Rules
 443The configuration file /etc/smack/accesses contains the rules to be set at
 444system startup. The contents are written to the special file /smack/load.
 445Rules can be written to /smack/load at any time and take effect immediately.
 446For any pair of subject and object labels there can be only one rule, with the
 447most recently specified overriding any earlier specification.
 449The program smackload is provided to ensure data is formatted
 450properly when written to /smack/load. This program reads lines
 451of the form
 453    subjectlabel objectlabel mode.
 455Task Attribute
 457The Smack label of a process can be read from /proc/<pid>/attr/current. A
 458process can read its own Smack label from /proc/self/attr/current. A
 459privileged process can change its own Smack label by writing to
 460/proc/self/attr/current but not the label of another process.
 462File Attribute
 464The Smack label of a filesystem object is stored as an extended attribute
 465named SMACK64 on the file. This attribute is in the security namespace. It can
 466only be changed by a process with privilege.
 470A process with CAP_MAC_OVERRIDE is privileged.
 472Smack Networking
 474As mentioned before, Smack enforces access control on network protocol
 475transmissions. Every packet sent by a Smack process is tagged with its Smack
 476label. This is done by adding a CIPSO tag to the header of the IP packet. Each
 477packet received is expected to have a CIPSO tag that identifies the label and
 478if it lacks such a tag the network ambient label is assumed. Before the packet
 479is delivered a check is made to determine that a subject with the label on the
 480packet has write access to the receiving process and if that is not the case
 481the packet is dropped.
 483CIPSO Configuration
 485It is normally unnecessary to specify the CIPSO configuration. The default
 486values used by the system handle all internal cases. Smack will compose CIPSO
 487label values to match the Smack labels being used without administrative
 488intervention. Unlabeled packets that come into the system will be given the
 489ambient label.
 491Smack requires configuration in the case where packets from a system that is
 492not smack that speaks CIPSO may be encountered. Usually this will be a Trusted
 493Solaris system, but there are other, less widely deployed systems out there.
 494CIPSO provides 3 important values, a Domain Of Interpretation (DOI), a level,
 495and a category set with each packet. The DOI is intended to identify a group
 496of systems that use compatible labeling schemes, and the DOI specified on the
 497smack system must match that of the remote system or packets will be
 498discarded. The DOI is 3 by default. The value can be read from /smack/doi and
 499can be changed by writing to /smack/doi.
 501The label and category set are mapped to a Smack label as defined in
 504A Smack/CIPSO mapping has the form:
 506        smack level [category [category]*]
 508Smack does not expect the level or category sets to be related in any
 509particular way and does not assume or assign accesses based on them. Some
 510examples of mappings:
 512        TopSecret 7
 513        TS:A,B    7 1 2
 514        SecBDE    5 2 4 6
 515        RAFTERS   7 12 26
 517The ":" and "," characters are permitted in a Smack label but have no special
 520The mapping of Smack labels to CIPSO values is defined by writing to
 521/smack/cipso. Again, the format of data written to this special file
 522is highly restrictive, so the program smackcipso is provided to
 523ensure the writes are done properly. This program takes mappings
 524on the standard input and sends them to /smack/cipso properly.
 526In addition to explicit mappings Smack supports direct CIPSO mappings. One
 527CIPSO level is used to indicate that the category set passed in the packet is
 528in fact an encoding of the Smack label. The level used is 250 by default. The
 529value can be read from /smack/direct and changed by writing to /smack/direct.
 531Socket Attributes
 533There are two attributes that are associated with sockets. These attributes
 534can only be set by privileged tasks, but any task can read them for their own
 537        SMACK64IPIN: The Smack label of the task object. A privileged
 538        program that will enforce policy may set this to the star label.
 540        SMACK64IPOUT: The Smack label transmitted with outgoing packets.
 541        A privileged program may set this to match the label of another
 542        task with which it hopes to communicate.
 544Smack Netlabel Exceptions
 546You will often find that your labeled application has to talk to the outside,
 547unlabeled world. To do this there's a special file /smack/netlabel where you can
 548add some exceptions in the form of :
 549@IP1       LABEL1 or
 552It means that your application will have unlabeled access to @IP1 if it has
 553write access on LABEL1, and access to the subnet @IP2/MASK if it has write
 554access on LABEL2.
 556Entries in the /smack/netlabel file are matched by longest mask first, like in
 557classless IPv4 routing.
 559A special label '@' and an option '-CIPSO' can be used there :
 560@      means Internet, any application with any label has access to it
 561-CIPSO means standard CIPSO networking
 563If you don't know what CIPSO is and don't plan to use it, you can just do :
 564echo -CIPSO > /smack/netlabel
 565echo @      > /smack/netlabel
 567If you use CIPSO on your local network and need also unlabeled
 568Internet access, you can have :
 569echo      -CIPSO > /smack/netlabel
 570echo -CIPSO > /smack/netlabel
 571echo      @      > /smack/netlabel
 574Writing Applications for Smack
 576There are three sorts of applications that will run on a Smack system. How an
 577application interacts with Smack will determine what it will have to do to
 578work properly under Smack.
 580Smack Ignorant Applications
 582By far the majority of applications have no reason whatever to care about the
 583unique properties of Smack. Since invoking a program has no impact on the
 584Smack label associated with the process the only concern likely to arise is
 585whether the process has execute access to the program.
 587Smack Relevant Applications
 589Some programs can be improved by teaching them about Smack, but do not make
 590any security decisions themselves. The utility ls(1) is one example of such a
 593Smack Enforcing Applications
 595These are special programs that not only know about Smack, but participate in
 596the enforcement of system policy. In most cases these are the programs that
 597set up user sessions. There are also network services that provide information
 598to processes running with various labels.
 600File System Interfaces
 602Smack maintains labels on file system objects using extended attributes. The
 603Smack label of a file, directory, or other file system object can be obtained
 604using getxattr(2).
 606        len = getxattr("/", "security.SMACK64", value, sizeof (value));
 608will put the Smack label of the root directory into value. A privileged
 609process can set the Smack label of a file system object with setxattr(2).
 611        len = strlen("Rubble");
 612        rc = setxattr("/foo", "security.SMACK64", "Rubble", len, 0);
 614will set the Smack label of /foo to "Rubble" if the program has appropriate
 617Socket Interfaces
 619The socket attributes can be read using fgetxattr(2).
 621A privileged process can set the Smack label of outgoing packets with
 624        len = strlen("Rubble");
 625        rc = fsetxattr(fd, "security.SMACK64IPOUT", "Rubble", len, 0);
 627will set the Smack label "Rubble" on packets going out from the socket if the
 628program has appropriate privilege.
 630        rc = fsetxattr(fd, "security.SMACK64IPIN, "*", strlen("*"), 0);
 632will set the Smack label "*" as the object label against which incoming
 633packets will be checked if the program has appropriate privilege.
 637Smack supports some mount options:
 639        smackfsdef=label: specifies the label to give files that lack
 640        the Smack label extended attribute.
 642        smackfsroot=label: specifies the label to assign the root of the
 643        file system if it lacks the Smack extended attribute.
 645        smackfshat=label: specifies a label that must have read access to
 646        all labels set on the filesystem. Not yet enforced.
 648        smackfsfloor=label: specifies a label to which all labels set on the
 649        filesystem must have read access. Not yet enforced.
 651These mount options apply to all file system types.
 653Smack auditing
 655If you want Smack auditing of security events, you need to set CONFIG_AUDIT
 656in your kernel configuration.
 657By default, all denied events will be audited. You can change this behavior by
 658writing a single character to the /smack/logging file :
 6590 : no logging
 6601 : log denied (default)
 6612 : log accepted
 6623 : log denied & accepted
 664Events are logged as 'key=value' pairs, for each event you at least will get
 665the subject, the object, the rights requested, the action, the kernel function
 666that triggered the event, plus other pairs depending on the type of event
 668 kindly hosted by Redpill Linpro AS, provider of Linux consulting and operations services since 1995.